Low Salt

by Nancy Gross

Brackish groundwater: The bad news is that it has higher levels of TDS than potable water, so you can’t just pump it out of the ground to include among drinking water resources. But the good news is that it has a lower concentration of TDS than seawater, which means that treating it is less energy-intensive and more cost-effective.

Brackish waters can be found in coastal areas (bays and estuaries, where fresh water mixes with salt water), in aquifers (where it is usually referred to as saline water), and in surface waters (salt marshes, for instance, contain brackish water). Brackish water sources produce a number of challenges for use:

* Water salinity allows for a broader range of applicable treatment technologies than seawater desalination

* Water composition can include large concentrations of sparingly soluble carbonate and silica salts that can cause scaling

* The affect of long term pumping of brackish ground water aquifers on fresh groundwater resources is unknown

A number of utilities in the West and Southwest are implementing brackish water desalination to augment their water supply. San Antonio Water System is a prominent example. You can read about their brackish desal program, beyond the paragraphs copied below, on the SAWS website:

San Antonio Water System is currently developing a brackish groundwater desalination program in southern Bexar County. Brackish groundwater is a plentiful, previously untapped local source of water that will help diversify San Antonio’s supplies.

SAWS future desalination facility will generate about 12 million gallons of water per day (mgd) or 13,440 acre-feet per year from the Wilcox Aquifer in Phase I. The plant will be located at the existing SAWS Twin Oaks Aquifer Storage & Recovery site.

The well sites will be located on adjacent SAWS property. Phases II and III will be completed in 2021 and 2026 respectively and will deliver a total of more than 30 mgd or 33,600 acre-feet per year. The total capital costs of the program for all three phases, including land acquisition, feasibility, design, construction and SAWS overhead is currently estimated at about $411.4 million. The cost per acre-foot of all three phases of the program is estimated at $1,138.